The Highlands Province

The region's most ancient rocks are the foundation of the Highlands region.
Metamorphic and igneous rocks of Late Precambrian and Early Paleozoic age crop
out in northeast-trending belts of rock forming the core of the Appalachian
Mountains in the New York Bight region. The area is a rugged, hilly to mountainous
terrain, bearing the characteristic scars of Pleistocene glaciation. The rocky
outcrops visible on hillsides, and along stream banks and roadside exposures,
consist mostly of gneiss, schist, and marble. The Manhattan skyline owes its
existence to the durable character of this bedrock foundation. Some of the most
scenic hiking areas in the region are found in the Highlands.

The literature is filled with conflicting definitions as to how the geographic
areas surrounding the New York Bight should be subdivided. The rocks of the
Highlands Province are a part of the "Crystalline Appalachians." This
name refers to the term crystalline rock, a general term for igneous and metamorphic
rock (as opposed to sedimentary rock) where the rock consists of minerals in
an obviously crystalline state. The Highlands region is equivalent to the ancient,
worn-down mountains of the Piedmont and Blue Ridge Provinces in North Carolina
and Virginia, and is equivalent to the New England Upland Province to the north.
Although the Newark and Connecticut River basins are equivalent to Triassic
age basins of the Piedmont to the south, for this discussion, the Highlands
Province is limited to exposures of Precambrian and Early Paleozoic metamorphic
and igneous rocks throughout portions of northern New Jersey, Southern New York,
and most of Connecticut.

The Highlands Province is subdivided into several regions (Figure 10). The
Reading Prong extends from southeastern Pennsylvania into the upland areas of
northern New Jersey where it is called the New Jersey Highlands. This belt continues
into southeastern New York where it is called the Hudson Highlands. Equivalent
rocks in western Connecticut are called the Housatonic Highlands. The Manhattan
Prong is a smaller belt of ancient rock in southern New York (including Manhattan,
the Bronx, and segments of Brooklyn and Staten Island), parts of Westchester
County, and upland portions of southwestern Connecticut.

Figure 10. The Highlands Province.

The Manhattan and Reading Prongs are separated by the Newark Basin in the south,
but the two features merge at the northern terminus of the Newark Basin in the
vicinity of Peekskill, New York. A band of mountains that rise nearly one thousand
feet along the northwestern margin of the Newark Basin in New York and New Jersey
are called the Ramapo Mountains. Another belt of ancient metamorphic and igneous
rock crops out along the southern margin of the Newark Basin south and west
of Trenton, New Jersey. In this region the rocks are referred to as part of
the Trenton Prong.

The Taconic Mountains consist of complexly folded and faulted Early Paleozoic
rocks which, in part, have not been subjected to the high degree of metamorphism
as rocks to the south and east. The Iapetus and Avalonian terranes in Connecticut
consist of intensely metamorphosed Precambrian and Early Paleozoic rocks, and
are defined, in part, by aspects of their geologic history (discussed below).

The geology of the Highlands Province is exceedingly complicated, and many
aspects of the region's geologic history are not clearly resolved. Detailed
geologic maps of the region show complex patterns of folds, faults, and intrusions.
These ancient rocks compose the basement beneath the younger, overlying sedimentary
strata of the Valley and Ridge Province, the sedimentary and volcanic rocks
of the Newark and Connecticut River Basins, and the poorly consolidated sediments
of the Coastal Plain. The rocks have been subjected to several episodes of tectonic
deformation, including stages of intense folding and metamorphism associated
with the collision of ancient landmasses driven by plate tectonic forces. Following
these mountain building episodes of the Paleozoic, this region has endured extensive
periods of gradual uplift and erosion lasting throughout Mesozoic and Cenozoic
time. As a result, the highest peaks in the Appalachian region generally consist
of ancient crystalline rocks that are more resistant to erosion. River valleys
generally follow along fractured zones and faults, or along outcrop belts of
rocks that preferentially weather and erode faster.

The Grenville Orogeny

The oldest exposures of Precambrian rock in the New York Bight region are estimated
at 1.3 to 1.1 billion years old (Middle Proterozoic). Whether older rocks existed
at one time in the region is unknown; they have long since been destroyed by
erosion, or have been incorporated deep into the crust or possibly back into
the mantle. The majority of these ancient rocks, however, have been subjected
to repeated episodes of metamorphism so that their radiogenic dates have been
reset to younger ages, mostly around 0.8 billion years ago. The early stages
of alteration occurred in an extensive mountain-building episode that affected
the entire eastern margin of North America extending as far west as Ohio and
Kentucky. This mountain-building episode is collectively known as the Grenville
Orogeny which began roughly 1.3 billion years ago (Middle Proterozoic), and
ended roughly around 800 million years ago (Late Proterozoic)(Figure 11A). During
this period older rocks and sediments deposited along the south and eastern
margin of the Canadian Shield underwent deep burial and metamorphic alteration.
Many igneous intrusions occurred in the region. Some of these igneous bodies
were probably sills or flows preserved within sequences of sedimentary rocks,
later to become metamorphosed during subsequent stages of the orogeny. Others
large magma chambers cooled slowly deep in the crust, becoming the granitic
core of many mountains in the Highlands region (such as Bear Mountain, New York).
Through much of their history these rocks were deeply buried, probably many
miles beneath the Earth's surface. Their overburden has long been stripped away
by erosion.

Figure 11. Plate tectonic configuration during Late Precambrian through
Early Paleozoic time for the New York City region (not to scale).

A long period of uplift and erosion followed the Grenville Orogeny. Beginning
roughly 700 million years ago rifting began along the eastern edge of the North
American continent (Figure 11B). This divergence (rifting) led to the formation
of the ancient Iapetus Ocean (a waterway that has since vanished by later collisions
between continents). According to Greek Mythology, Iapetus was the father of
Atlas, the name-sake of the Atlantic Ocean.
Through late Proterozoic into Cambrian time, sediments derived from upland areas
to the north and west accumulated along a continental shelf margin and within
a deep ocean trough farther offshore. The shallow shelf margin followed a general
trend along the southeastern edge of the New Jersey, Hudson , and Housetonic
Highlands. On the shallow shelf regions well-sorted quartz sandstone and carbonate
rocks (limestone and dolomite) were deposited. This continental shelf sequence
accumulated to the thickness of several miles, forming an elongated sedimentary
basin referred to as a miogeocline (Figure 11C). To the east, a deep water trough
filled with fine-grained sediments (mainly shales) derived from both the continent
and from volcanic island arc systems that were emerging far offshore within
the Iapetus Ocean basin. This deep water basin sequence is referred to as a
eugeocline. To the south and east, this basin was underlain by basaltic crust
of the ancient Iapetus Ocean (see Figure 11C). Sedimentation in this basinal
trough persisted until Middle Ordovician time.

The Taconic Orogeny

The Taconic Orogeny was a great mountain building period that perhaps had
the greatest overall effect on the geologic structure of basement rocks within
the New York Bight region. The effects of this orogeny are most apparent throughout
New England, but the sediments derived from mountainous areas formed in the
northeast can be traced throughout the Appalachian and Midcontinent regions
of North America. The following discussion provides a summary of events leading
to the culmination of this orogeny.

Beginning in Cambrian time (about 550 million years ago) the Iapetus Ocean
began to grow progressively narrower. The weight of accumulating sediments,
in addition to compressional forces in the crust, forced the eastern edge of
the North American continent to gradually fold downward (Figure 11D). In this
manner, shallow carbonate deposition that had persisted on the shelf margin
through Late Cambrian into Early Ordovician time, gave way to fine-grained clastic
deposition and deeper water conditions during the Middle Ordovician. Sometime
during this period a convergent plate boundary developed along the eastern edge
of a small island chain. Crustal material beneath the Iapetus Ocean sank into
the mantle along a subduction zone with an eastward-dipping-orientation. Partial
melting of the down-going plate produced magma that returned to the surface
to form the Taconic island arc offshore from the continent. By the Late Ordovician,
this island arc had collided with the North American continent. The sedimentary
and igneous rock between the land masses were intensely folded and faulted,
and were subjected to varying degrees of intense metamorphism (Figure 11E).
This was the final episode of the long-lasting mountain-buliding period referred
to as the Taconic Orogeny.

When the Taconic Orogeny subsided in the New York Bight region during Late
Ordovician time (about 440 million years ago), subduction ended, culminating
in the accretion of the Iapetus Terrane onto the eastern margin of the continent.
This resulted in the formation of a great mountain range throughout New England
and eastern Canada, and perhaps to a lesser degree, southward along the region
that is now the Piedmont of eastern North America. The newly expanded continental
margin gradually stabilized. Erosion continued to strip away sediments from
upland areas. Inland seas covering the Midcontinent gradually expanded eastward
into the New York Bight region and became the site of shallow clastic and carbonate
deposition. This tectonically-quiet period persisted until the Late Devonian
time (about 360 million years ago) when the next period of mountain-building
began, the Acadian Orogeny.

Major Rock Groups

Rock units of Precambrian and Lower Proterozoic Age have been subdivided into
numerous formations and members, with different names applied to equivalent-age
rock units from one region to the next. More detailed descriptions of local
and regional stratigraphy are available in many reports and maps cited in the
reference section of this book. The list of rock groups in Figure 12 is both
generalized and incomplete; however, it highlights the major rock groups that
crop out within the New York Bight region. More detailed discussion of some
of these units are incorporated into the following locality descriptions.

Figure 12. Generalized Precambrian and Lower Paleozoic stratigraphy
for the New York City region (modified after American Association
of Petroleum Geologists COSUNA Charts (1983, 1986), Drake and others
(1986), Fisher and others (1995), and Rodgers (1985).

When traveling across the region from the north and west toward the south and
east the rocks of lower Paleozoic age display increasing stages of metamorphism.
This is because this region was subjected to higher degree of deformation associated
with the Taconic Orogeny. For instance, the Late Cambrian and Early Ordovician
limestones and dolomite of the Kittatiny Group in northwestern New Jersey are
equivalent to the Inwood Marble in the Manhattan Prong region. Likewise, the
Late Ordovician Martinsburg and Normanskill shales are equivalent to the Manhattan
Schist. All rocks in the region formed prior to or during the Taconic Orogeny
were also subjected to additional deformation episodes during the Acadian and
Alleghenian Orogenies later in the Paleozoic.

Mountains Without "Roots"

During the process of continental collision and accretion, rocks in the crust
are subjected to great forces of pressure. They eventually yield to this pressure
by bending or fracturing. If the rock is confined under great pressure they
will flow gradually like plastic, resulting in the formation of folds (anticlines
and synclines [Figure 13]). Such evidence of plastic deformation can be observed
on the scale of a small hand specimen or traced to a series of folds on a scale
of tens or even hundreds of miles. If the pressure is applied too rapidly the
rocks will behave in a more brittle manner forming fractures (joints and faults).
A joint differs from a small fault in that a fault displays evidence of visible
offset. In many cases faults are only observed on the surface as a relative
change in lithology from one surface outcropping to the next.

Figure 13. Types of faults and folds.

Though time, rocks respond to crustal pressure by folding or moving passively
past one another along faults. Earth movement is typically an extremely slow,
gradual process. However, when pressure builds beyond the point to which rocks
can gradually yield they may suddenly break and move, causing an earthquake.
Earthquakes are measured by different levels of intensity (see additional discussion
in the Human Impacts chapter).
Most earthquakes are imperceptible to humans; only great quakes are capable
of causing damage and creating visible changes on the Earth's surface. Earth
displacements measured following the greatest earthquakes in modern times are
only measured in the order of several dozens of feet.

During the mountain-building episodes during the Paleozoic, great piles of
thrust sheets piled one on another, built up into mountain ranges along the
eastern margin of North America. As mountain building proceeded, certain faults
may have been active for a time, then motion may have ceased as active displacement
occurred along new fault systems or along other older fracture zones. In many
fault systems relative motion is of strike-slip displacement, where one rock
mass to move horizontally past another, in some cases for great distances. Over
the course of geologic time, fault systems may become reactivated with relative
motion changing from one episode to the next. Several major thrust fault systems
are known throughout the New York Bight region; some display a large degree
of strike-slip offset.
Perhaps the greatest fault system in the New York Bight region is known as Cameron's
Line, named after Eugene N. Cameron, a U.S. Geological Survey geologist who
first recognized its significance as a great suture between rocks of widely
different origins. Cameron's Line winds southward out of New England into Western
Connecticut and passes through southern New York across the Bronx, following
the general trend of the East River. It extends beneath sedimentary cover on
Staten Island and southward beneath the Coastal Plain of New Jersey. In general,
basement rocks to the west of Cameron's Line are regarded as autochthonous,
meaning that they have not been significantly displaced by tectonic processes.
The rocks to the west of Cameron's Line include metamorphosed sedimentary material
originally comprising ancient continental slope, rise, and shelf deposits, collectively
described as a miogeocline (see Figure 11C and Figure 12). The rocks to the
east of Cameron's Line are allochthonous, which means they have been shoved
westward over autochthonous basement rocks on the order of many tens or even
hundreds of kilometers. These rocks were originally deposited as sediments in
a deep water basin called a eugeocline. Cameron's Line may represent the trace
of a subduction zone that ceased when the smaller Taconic Landmass, a small
terrane consisting of oceanic crust and a complex island arc system, collided
with, and became accreted onto, the eastern margin of North America during the
Taconic Orogeny. Much of the rocks east of Cameron's Line were once part of
the floor of the Iapetus Ocean (the Iapetus Terrane).

Rocks to the west of Cameron's Line have also been displaced along great thrust
faults. These faults generally dip steeply near the surface, but decrease to
shallow angles in the subsurface. As with most thrust faults in the region,
older metamorphic rocks have been shoved westward over younger sedimentary rocks
of Cambrian and Ordovician age. In many areas there are numerous thrust sheets.
The term, imbrication, is used to describe great massifs of rock in which sequences
of stratigraphic intervals are repeated in stacked vertical succession. This
is particulary evident in the Taconic Mountains, throughout the Hudson Valley,
and in portions of the New Jersey Highlands. In several locations of the Highlands
region the upland areas are underlain by Precambrian gneisses whereas the exposures
along the stream valleys consist of Cambrian and Ordovician shales and limestones.
The Precambrian gneisses have been shoved north and west over the younger sedimentary
rocks along great thrust faults. The occurrence of older, more resistant crystalline
rocks forming the core of a mountain, with an adjacent valley underlain by softer
sedimentary rocks gives the impression of "mountains without roots."

Field Trips Sites In The Highlands

The Highlands Region is host to a wide variety of geology field trip destinations.
The localities described below were chosen for their geologic characteristics,
their scenic setting, and their general accessibility on public lands. All sites
are within about a two hour drive from practically anywhere in metropolitan
area (excluding traffic!). In most field location there are usually many geologic
features to look at: the composition and character of the bedrock; geologic
structures (such as folds or faults); weathering, erosion, and glacial features;
and in most cases, scenic views of the regional landscape. All of these components
provide the observer with evidence for the rich geologic history of a location.
Knowledge of the bedrock geology is essential for understanding aspects of the
natural ecology and the cumulative and changing history of human land use.

Although many classic geologic localities occur along road cuts in the region
they have been intentionally excluded from this collection because it is both
unsafe and illegal to stop along most highway exposures. Some of the most spectacular
outcrops in the Highlands region occur along the New York Thruway, Merrick Expressway,
Cross Bronx Expressway, the Taconic Parkway, I-287, and many other highways
in the region. Unfortunately, curiosity-driven rock-hounds have caused highway
accidents, some resulting in major traffic jams; even deaths have occurred.
Fortunately, as the discussion below illustrates, there are many places where
classic exposures are preserved and protected. In general, rock collecting is
strictly prohibited on park lands (with stiff fines!), however, there is no
shortage of places where fossils, rocks, and minerals may be collected both
legally and safely. Collecting is best when it is done with a group of knowledgeable
enthusiasts during an organized field trip. The best method of collecting, however,
is with a camera!

Highlands Sites Within New York City

Accessible outcrops of crystalline basement rocks occur throughout parklands
in Manhattan, the Bronx, and Staten Island. Rubble from construction of buildings
and tunnels are scattered throughout the city, used in the construction of seawalls,
groins, and jetties, as well as in many buildings and infrastructure throughout
the city. The localities included below are only a partial selection of potential
study sites. They have been chosen because many are well known or "classic"
sites that have been examined and re-examined throughout the history of geologic
investigations of the city's bedrock foundation. Public locations within New
York City that feature Highlands geology discussions include:

The Hudson Highlands region represents the outcrop belt of crystalline basement
rocks of the northeastern extension of the Reading Prong (see Figure 10). The
Hudson Highlands are a northern extension of the Ramapo Mountains in northern
New Jersey. To the north and east, the boundary between the Hudson Highlands,
the Taconic Mountains, and the Western Connecticut Uplands region is indistinct,
partly because of the occurrence of numerous thrust faults and large igneous
Middle Paleozoic intrusions in the region. In general, the name Hudson Highlands
refers to the low mountains or high hills that border the Hudson River north
of Peekskill and south of Newburgh, New York. Links to Highlands in the Hudson
River region include:

Of the choices to drive north and east from New York City into Connecticut,
the Merritt Parkway provides a more scenic alternative over I-95. There are
numerous spectacular roadside exposures of dark gneisses and schists with light
colored veins of quartz, migmatite, and pegmatite. Unfortunately, these outcrops
are nowhere safe for casual roadside examination. Suitable exposures for study
might be found along exits on the parkway. One suitable field site in the Connecticut
Panhandle is: 12. Audubon Center.

The Housetonic Mountains in northwestern Connecticut are a wonderful destination
for a long day trip or a weekend outing. There are numerous state parks, and
roadside picnic areas along scenic highways. US Highway 7 is a major scenic
route leading north from Danbury into this mountainous region with numerous
peaks rising above 1,500 feet. Mt. Fissell, the highest peak in Connecticut
located at the northwestern corner of the state reaches an elevation of 2380
feet. Two places to consider visiting first are:

New Jersey has perhaps the most varied and accessible public lands within the
Highland Province. The selection below provides some of the best know geologic
wonders in the region. In addition to being host to interesting rocks and minerals,
all these localities provide unique information about earth history relating
to the formation of the North American continent from the Late Precambrian through
the Paleozoic Era, and also provide spectacular examples of glacial features
and modern landscape development. Discussion and sites in the New Jersey Highlands
include: